1. Field of the Invention
This invention relates to a 1-μm fiber Amplified Spontaneous Emission (ASE) source.
2. Description of the Related Art
Fiber ASE sources are broadband light sources based on the emission properties dictated by the energy structure of dopant ions in the glass host. A pump laser energizes the dopant ions so that spontaneously emitted light from one ion propagates along the fiber where it is amplified by the gain properties of the fiber and emitted as ASE. Light is emitted in both forward and backward directions, relative to pump direction, but the backward ASE, or counter-pumped direction, has a higher quantum efficiency and is selected as the source output. Unlike lasers, ASE sources do not rely on optical feedback, and thus the full-width half-maximum (FWHM) bandwidth of the backward ASE is generally very broad, typically between 10 and 30 nanometers, with a very short coherence length, typically less than 50 μm.
The relation between coherence length and FWHM bandwidth is given by:Δz=ln(2)(2/π)(λ2/Δλ)Where λ is the source mean wavelength, Δλ is the FWHM bandwidth, and Δz is the coherence length. The wider the FWHM bandwidth the shorter the coherence length.
ASE sources are widely used for test and measurement in such fields as optical spectroscopy of materials, optical component testing, optical coherence tomography, and optical fiber gyroscopes. The incoherent light source enables measurement of insertion loss, crosstalk, bandwidth, polarization dependencies, and other parameters of passive optical components versus wavelength. The most common fiber ASE source comprises a single-mode pump that energizes a length of Er-doped single-mode silica fiber, typically tens of meters, to emit at 1.5 μm. In these ASE sources there is a tradeoff between (1) power and (2) emission bandwidth (coherence length) and spectral stability/purity. If the source is configured for high power, the bandwidth will be narrower, hence coherence length longer and the spectral properties will suffer. Much effort has been made to provide fiber ASE sources that provide all of these properties. In addition, some applications require not only a short coherence length but an emission bandwidth that covers a different range of wavelengths than is supported by conventional sources. For example, in ophthalmic OCT two separate light sources centered at 850 nm and 1300 nm are used to perform retinal and corneal scans, respectively. It would be very useful to have a single light source with the power, penetration depth, coherence length and bandwidth capability to perform both tests simultaneously.
A stable, broad-band two-stage superfluorescent source at 1.55 μm was demonstrated using an erbium-doped fiber (EDF) seed source and a high-power Er—Yb fiber amplifier. The source exhibited from 140 to 220 mW of power, 18 to 28 nm bandwidth, with an estimated mean wavelength stability from 1 to 10 ppm. The use of second light source to “seed” the primary gain fiber with forward ASE enabled the source to provide the high power, large bandwidth and stable wavelength reported. (See Dagenais et al. “Wavelength Stability Characteristics of a High-Power, Amplified Superfluorescent Source” Journal of Lightwave Technology. Vol. 17, No. 8, pp. 1415-1422 August 1999.
A 75-nm, 30-mW superfluorescent ytterbium fiber source operating around 1.06 μm was reported by Chernikov et al. “A 75 nm, 30-mW superfluorescent ytterbium fiber source operating around 1.06 μm”, Conference on Lasers and Electro-Optics (CLEO), Paper CTuG8, 1997). The source is configured with three sections of Yb-doped germano-silicate fiber. The first section (amplifier) is pumped to provide backward ASE at 1 μm with a 40-nm spectrum. The third section is pumped to seed the amplifier, which broadens the spectrum to as much as 60-nm. The second inner section is unpumped and used with a spectrally optimized mirror M1 to provide spectrally controlled feedback to achieve a spectral width up to 76 nm. By reducing the FWHM bandwidth, the source can output a smooth Gaussian-like spectral shape that is preferred for such applications as OCT. This approach provides a broadband 1-μm fiber ASE source centered at 1060 nm but is complicated by the unpumped fiber and spectrally tailored mirror, which make the source less stable and more expensive. Furthermore, to achieve the spectral shape desired for OCT bandwidth must be sacrificed. Although the bandwidth is broad, when centered at 1060 nm the source is unable to reach certain shorter wavelengths below 1040 nm. Furthermore, photodarkening limits the doping % to less than about 0.2-0.3 wt. %. Consequently to achieve the desired wavelengths and output powers the silica based fiber is typically tens of meters in length.